Messages reportedly sent from a pupil in the school, and shared on social media, said "some seniors tackled the gunmen" before adding there was "blood on the floor".
Josh Dutton, 18, told AP news agency he used to be friends with Mr Erickson at a former school but had not seen him in four years.
On Tuesday, two suspects shot up the STEM School Highlands Ranch, killing at least one student and injuring eight more.
The STEM school, which is kindergarten through 12th grade, was on lockdown Tuesday afternoon and all other Douglas County schools were on lockout, the school district said.
Two surviving victims of the attack at the Science, Technology, Engineering and Math (STEM) School in Highlands Ranch, a Denver suburb, remained in serious condition, medical officials said. The two schools are separated by about 7 miles (11 kilometers) in adjacent communities south of Denver. She said he was such a terrific employee they offered him a part-time job.
Mr Spurlock said the school contacted the emergency services "almost immediately", and that officers arrived at the scene about two minutes later.
"That's when Kendrick lunged at him and he shot Kendrick, giving all of us enough time to get underneath our desks, to get ourselves safe and to run out of, across the room to escape", Nui Giasolli, a fellow student, told WCNC.
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As the gunfire rang out, students ran through the halls shouting "School shooter!".
The shooters entered through the middle school entrance, which is in the northeast side of the school, Spurlock said.
There were around 1,800 students on campus at the time of the attack, Sheriff Spurlock said. Devon Erickson, 18, was named as one of the suspects, while the other was identified only as a juvenile.
"He said a guy pulled a pistol out of a guitar case and started to shoot", Montoya told the Denver TV station.
The sheriff confirmed seven student injuries, possibly eight, and two suspects are in custody.
Sophia Marks' mother, Sara Marks, said she has two other children who also go to the school. Nearby, Columbine High School-where, on April, 20, 1999, students Dylan Klebold and Eric Harris murdered 12 students and 1 teacher before committing suicide-was placed on alert.
Department of Homeland Security, FBI and ATF agents also responded to the school to help with the investigation.
In 1999, two students killed 13 people at Columbine High School before committing suicide in what remains one of the deadliest school shootings in U.S. history.